seasonal circulation on the western shelf of the gulf of mexico using a high-resolution numerical...

7/17/2019 Seasonal circulation on the western shelf of the Gulf of Mexico using a high-resolution numerical model

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Seasonal circulation on the western shelf of the Gulf of Mexico using a

high-resolution numerical model

Jorge Z avala-Hidalgo,1 S t ev e n L . M o re y, a n d J a me s J . O Br i enC e n t er f o r O c e an - A t mo s p he r i c P r e di c t i o n S t u d i e s , F l o r id a S t a t e U n i v er s i t y, Ta l l a h as s e e , F l o r id a , U S A

R e c ei v e d 3 1 M a r ch 2 0 0 3; r e v i se d 1 8 J u l y 2 0 0 3 ; a c c ep t e d 2 3 S e p te m b e r 2 0 0 3 ; p u b l i s h ed 2 6 D e c em b e r 2 0 0 3 .

[1] T h e s e a so n al c i rc u la t io n o n t h e w e st e rn s h el f o f t h e G u lf o f M e xi c o i s s tu d ie d u s i n g ah i g h - re s o lu t i o n n u m er i c a l s i m u l a t io n , h i s to r i c al h y d r o gr a p h ic d a t a, s e a l e v el d a t a , a n ds a t e ll i t e i m a ge s . T h r e e r e g i on s a r e d i s t in g u i sh e d , t h e Ta m a ul i p a s- Ve r a c ru z ( TAV E ) s h e l f,t h e L o u i s i a n a - T e x a s ( L A T E X ) s h e l f , a n d t h e w e s t e r n C a m p e c h e B a n k . O n t h e T A V E s h e l f t h er e i s a s wi f t r e ve r sa l o f t h e a l on g -s h el f c u rr e nt , d o wn c oa st f r om S e pt e mb e r t o M ar c ha nd u pc oa st f ro m M ay t o A ug us t w he n t he re i s u pw el li ng d ue t o o ff sh or e E km ant r an s po r t. C i rc u la t io n o n t h e w e st e rn C a mp e ch e B a nk i s u p co a st t h ro u gh o ut t h e y e ar. T h eL AT E X s h el f h a s a c y cl o ni c c i rc u la t io n , e x ce p t d u ri n g s u mm e r m o nt h s w h en t h e f l ow i s

e a st w ar d . D u ri n g s p ri n g- s um m er t h e u p co a st c u rr e nt o n t h e TAV E s h el f r e ac h es t h es o ut h er n Te x as s h el f w h er e i t e n co u nt e rs a d o wn c oa s t c o as t al c u rr e nt f a vo r in g o f fs h or et r a n s p o r t s . I n t h e f a l l - w i n t e r , t h e d o w n c o a s t c u r r e n t r e a c h e s t h e s o u t h e r n B a y o f C a m p e c h ew h e r e i t m e e ts a n o p p o si n g a l o n g- s h el f c u r r e nt , g e n e ra t i n g s e a s o n al o f f sh o r e t r a n sp o r t s.D u ri n g f a ll a n d w i nt e r, c o ol l o w- s al i ni t y w a te r f r om t h e M is s is s ip p i a n d A tc h af a la y aR i ve r s i s a d ve c te d w e st w ar d a l on g t h e L AT E X s h el f o n to t h e TAV E s h el f , d e ve l op i nga l o n g - s h e l f f r o n t s a n d t e m p e r a t u r e i n v e r s i o n s c o m m o n l y o b s e r v e d o v e r t h e o u t e r s h e l f a n ds he lf b re ak . T he m ai n f or ci ng o ve r t he w es te rn s he lf o f t he g ul f i s t he a lo ng -c oa st w in ds t re s s c o mp o ne n t. T h e e x is t en c e o f t h e c r os s -s h el f t r an s po r ts i n t h e c o nf l ue n ce r e gi o ns i ss u p p or t e d b y c h l o ro p h y ll a d a ta . U p t o 8 0% o f t he s ea so na l s ea l ev el v ar ia bi li ty i se x p l a i n e d b y t h e a l o n g s h e l f c u r r e n t s a n d t h e l o w - f r e q u e n c y v a r i a b i l i t y o f t h e a t m o s p h e r i cs e a l e v e l p r e s s u r e . INDEXTERMS: 4 2 2 7 O c e a n o g r a p h y : G e n e r a l : D i u r n a l , s e a s o n a l , a n d a n n u a l c y c l e s ;4 2 1 9 O c e a n o gr a p h y : G e n e r a l : C o n t i n e n ta l s h e l f p r o c e s s e s ; 4 2 2 3 O c e a n og r a p h y : G e n e r a l : D e s c r i p t iv e a n d

r e g i o n al o c e a n o gr a p h y ;KEYWORDS: s h e l f c i rc u la t io n , w e s te r n G u lf o f M e xi c o

Citation: Zavala-Hidalgo, J., S. L. Morey, and J. J. OBrien, Seasonal circulation on the western shelf of the Gulf of Mexico using a

high-resolution numerical model, J. Geophys. Res., 108( C 1 2 ) , 3 3 8 9 , d o i : 10 . 1 0 2 9 / 20 0 3 J C 0 01 8 7 9 , 2 0 0 3 .

1. Introduction

[2] C o ns i de ri n g t h e c on t in en t al p l at f or m w e st o f 8 9. 5Wa s t he w es te rn s he lf of t he G ul f of M exi co ( GoM ), i t i n cl u de s t h e L ou i si a na - Te xa s ( L AT E X) s he l f a n d t h es h el v es o f t he M e xi c an s t at e s o f Ta m au l ip as , Ve r ac r uz ,Ta ba sc o, C am pe ch e, a nd p ar t o f Yu ca ta n, u p t o P ue rt oP ro gr es o. D ef in in g t he o ut er s he lf b ou nd ar y a t 2 00 m ,

t he w est er n sh el f h as a n ar ea o f 26 7, 000 k m2

a nd i tsl e n gt h , d i s re g a rd i n g f e a t u re s s m a ll e r t h a n 5 0 k m , i s a r o un d2 , 50 0 k m ( Fi g ur e 1 ).

[3] A lt ho ug h t he c ir cu la ti on o n t he L AT EX s he lf a t t hes e a so n al f r e qu e n cy i s w e l l k n o w n , t h e r e g io n i s i n c lu d ed i nt h is s t ud y b ec a us e i t i s f o un d t ha t t he r e i s a s t ro ng r e la t io n

between its dynamics and that of the Mexican shelf. Itsi n c l u s i o n i s a l s o u s e f u l t o v a l i d a t e t h e m o d e l r e s u l t s b e c a u s et he a mo un t o f d at a a nd n um be r o f s tu di es a re l ar ge r h er ec o mp ar e d t o t ho s e o n t he M ex i ca n s he l f. O n t h e L AT EX

s h el f t h e d om i na nt p at t er n i s a c yc l on i c c i rc u la t io n w i th aw i nd -d r iv en d ow n co a st ( i n t h e d ir e ct i on t ha t t he K e lv inw a v e t r a v e ls ) j e t n e a r t h e c o a s t a n d a w e a ke r n o r th e as t w ar dc u rr e nt o n t h e o u t er s h el f [Cochrane and Kelly, 1 98 6 ;Oey,1995; Li et al., 1 9 9 6 ;Cho et al., 1 9 9 8 ;Nowlin et al., 1 9 9 8 ] .T hi s p at t er n p re v ai l s m o st o f t h e y e a r e xc e pt f r om J un e t oA u gu s t w h en t he s u rf a ce c u rr e nt s r u n u pc oa s t o ve r a l mo s t t he e nt ir e s he lf , e xc lu di ng s om e r eg io ns n ea r t he c oa st

m os tl y o n t he e as te rn s id e o f t he s he lf . T he m ai nl y w in dd r iv en n ea r sh or e f l ow i s e nh an ce d b y t he M i ss is s ip pi -A t c ha f a la y a R i v er s d i s ch a r ge [Li et al., 1 9 97 ] . T h e c o n c a ves ha pe o f t he c oa st a nd t he w in d s tr es s f or ci ng c au se ac o n ve r ge n c e o f c u r re n t s o n t h e w e s t e r n s i d e o f t h e L AT E Xs h el f a n d a d i ve rg en c e o n t he e as t er n s i de . Cochrane and

Kelly [ 1 9 86 ] p r o po s e d t h a t t h e c o nv e rg e n ce a n d d i v er g e nc ec o m p l e t e s t h e g y r e a n d d r i v e s t h e s o u t h e r n l i m b o f t h e g y r e .Alternatively,Oey [ 1 9 9 5 ] p r o p o s e d t h a t t h e s o u t h e r n l i m b o f t he g yr e m ay b e d ri ve n b y m es os ca le e dd ie s n ex t t o t hes he lf b re ak , a nd t he s tu di es f ro m t he L AT EX p ro gr am[Nowlin et al., 1 99 8] s ho w ed t h at t h e o u t er s he l f i s m a in lyd ri ve n b y t he m es os ca le e dd ie s t ha t h av e n ot a nn ua l

periodicity. The convergence at the western end of the gyrem i gr at e s s ea so na ll y, r e ac hi ng s o ut h o f t he R i o G r an de

J O U R N AL O F G E O P H YS I C A L R E S E A R CH , V O L . 1 0 8 , N O . C 1 2 , 3 3 8 9 , d o i : 1 0 . 10 2 9 / 2 0 03 J C 0 0 18 7 9 , 2 0 0 3

1 No w at Cen tro d e Cien cias d e la Atmosfera, Un iv ersid ad Nacio n alAuto noma de Mexi co, Mexi co, Mexi co.

C o p yr i g h t 2 0 0 3 b y t h e A m e ri c a n G e o p h y s ic a l U n i o n.0148-0227/03/2003JC001879$09.00

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m ou th i n f al l. M os t o f t he y ea r, a lo ng -s ho re i so ha li ne sprevail with lower salinity inshore, but during July andA u gu st , c ro ss - sh el f i s oh al i ne s a re o bs er ve d w i th l ow e r s al in it y i n t he e as t [Cochrane and Kelly, 1 98 6; Li et al.,1997; Nowlin et al., 1 9 98 ]. I n te ra n nu a l v a ri a bi l it y i n t heL AT EX s he l f c i rc u la t io n h as b e en d oc um e nt e d, w h ic h i s

particularly noticeable during the periods when the currentsr e v er s e s b e c a u se t h e c u r re n t c a n f l o w i n o p p os i t e d i r e c t io n si n d i ff er e nt y ea r s a s Cho et al. [ 1 99 8 ] a n d Nowlin et al.

[ 1 9 98 ] s h ow e d t h a t h a p p e ne d d u r in g t h e p e r io d f r o m A p r i l1 9 9 2 t o N o v em b e r 1 9 9 4 .

[4] T he d yn a mi c s o f t h e s h el f s o u th o f t h e U n i t ed S ta t es -M e xi c an b or d er a re l e ss k no w n. M os t o f t he s t ud ie s i n t hew e st e rn G oM h a ve b ee n d ed i ca t ed t o p ro c es s es t ha t o cc ur m a i nl y o f f sh o r e f r o m t h e s h e l f : e d d ie s a n d t h e i r i n t er a c ti o nw it h t he s lo pe [Merrell and Morrison, 1 98 1; Brooks and

Legeckis, 1 98 2 ;Elliott, 1 98 2; Merrell and Vazquez, 1 98 3;Vidal et al., 1 99 4a ], a nd t he w es te rn b ou nd ar y c ur re nt [Sturges and Blaha, 1 97 6; Blaha and Sturges, 1 98 1;Sturges, 1 99 3] . S om e o f t he se p ro ce ss es m ay a ff ec t t hed y n a m i c s o n t h e s h e l f , s u c h a s t h e L o o p C u r r e n t e d d i e s t h a t d e ca y n ex t t o t he s l op e a nd a f fe c t t h e c u rr e nt s i n t h e o u t er

s h e lf , t h e j e t s a s s oc i a te d w i t h e d d y p a i rs t h a t d e v e l op c r o ss -s he lf t r an sp or ts , a nd t he w es te rn b ou nd ar y c ur re nt t ha t

a ff ec ts t he c oa st al s ea l ev el a nd t he o ut er s he lf . I n a ne xc el le nt r ev ie w o f t he d yn am ic s o f t he s he lv es i n t hee a st e rn A t la nt i c c oa s t a n d t h e G o M, Boicourt et al. [1998]s ug ge st t ha t o n t he M ex ic an s he lf t he re i s a c ur re nt t ha t s ea so na ll y r ev er se s. T he y m ad e t hi s c on je ct ur e o n t he

basis of the analysis of AVHRR images and coastal zonec o lo r s c an n er i m ag es b y Biggs and Muller-Karger [1994]

t h at s u gg es t t ha t t he f al l -w i nt e r c oa s ta l c ur r en t o bs e rv e da lo ng t he L AT EX s he lf c on ti nu es a t l ea st a s f ar s ou th a sTa m pi c o, i n s o ut h er n Ta m au l ip a s, a n d o n d at a r e po r ts o f c ur re nt s 3 0 k m o ff sh or e Tu xp an ( 21 .6N, 97.1W ) a ndVe ra cr uz P or t t ha t s ho w d ow nc oa st c ur re nt s i n f al l a ndw in te r a nd u pc oa st c ur re nt s i n s pr in g a nd s um me r [Gutierrez de Velasco et al., 1 99 2, 1 99 3] . T he a n al y si s o f s u r f a c e d r i f t e r s d e p l o y e d i n 1 9 9 8 s h o w s a d o w n c o a s t c u r r e n t a l o n g t h e M e x i c a n s h e l f d u r i n g t h e f a l l [Lugo-Fernandez etal., 2 0 0 1 ] . S t u d i e s i n t h e B a y o f C a m p e c h e h a v e s h o w n t h a t t h e r e i s a m e a n c y c l o n i c c i r c u l a t i o n [Vazquez, 1 9 9 3 ] t h a t h a sa s ea so na l c yc le [Monreal et al., 1 99 2] , b ut t he y d id n ot s t u dy s p ec i f ic a l ly t h e s h e lf d y na m i c s. H o w ev e r, i t h a s b e en

reported that on the southern Tamaulipas-Veracruz (TAVE)s h el f t he w a te r c o lu m n i s m i xe d i n w i nt e r a n d s t ra t if i ed i ns u m m e r [Soto and Escobar, 1 9 9 5 ] . O n t h e C a m p e c h e B a n k ,a w es tw ar d c ur re nt h as b ee n s ug ge st ed [Monreal et al.,1992; Merino, 1 9 9 7] .

[5] T hi s p ap er d es c ri be s t h e m a i n c h ar a ct e ri s ti c s o f t hes e as o na l c i rc u la t io n o n t h e w e st er n s he l f o f t h e G o M a ndd i sc u ss e s t h e d y na m ic s o f t he c i rc ul a ti o n f e at u re s . T h iss tu dy i s b as ed o n t he r es ul ts o f a v er y h ig h- re so lu ti onn u m e r i c a l s i m u l a t i o n o f t h e e n t i r e G o M , a n d o n t h e a n a l y s i so f h y dr o g ra p hi c , s e a l e v el , AV H R R , a n d S e aW i FS d a t a. I ns e ct i on 2 w e b r ie fl y d es c ri b e t h e n um e ri c al m o de l a nd t hea na ly ze d d at a. R es ul ts a re p re se nt ed i n s ec ti on 3 , a nds e c ti o n s 4 a n d 5 i n c lu d e s o m e d i s c u ss i o n a n d c o n cl u s io n s.

2. Model and Data

[6] I n t hi s s tu dy, T he N av y C oa st al O ce an M od el ( NC OM )i s u se d t o s im ul at e t he G oM [Martin, 2 00 0; Morey et al.,2 00 3a ] . T he N C OM i s a p r im i ti v e e qu at i on h y dr os t at icm o de l w i th t h e B o us s in e sq a pp r ox im a ti on . I t i s s i mi l ar i ni ts p hy si cs a nd n um er ic s t o t he P ri nc et on O ce an M od el( P OM ) [Blumberg and Mellor, 1 98 7] , b ut h as s om e a dd i-t i o na l p h ys i c al a n d n u m er i c a l o p t i o n s. A r e m ar k a bl e d i f fe r -e nc e f ro m t he P OM i s t he u se o f h yb ri d c oo rd in at es t ha t a ll ow s t he u se o f s ig ma l ay er s n ea r t he s ur fa ce a nd g eo -

potential (z) level vertical coordinates below a specified

d e p th . T h es e c h a ra c t er i s ti c s m a k e t h e N C O M a g o o d m o d e lf o r t h e s t u d y o f c o as t a l c u r r e nt s , r e g io n s w i t h l a r ge s a l in i t ya n d t e m p e ra t u re g r a di e n ts , a n d t h e i n f lu e nc e o n t h e s h el f o f t h e r i c h e d d y f i e l d o f t h e o f f s h o r e e n v i r o n m e n t i n t h e G o M .T h e m o d e l d o m a i n i n c l u d e s t h e w h o l e G o M a n d t h e w e s t e r nCaribbean Sea, from 98.15W t o 8 0 . 6 0W a n d f r o m 1 5 . 5 5Nt o 3 1 . 5 0N, with model equations discretized on a 1/20 gridi n l a ti t ud e a nd l on gi t ud e. T he r e a r e 2 0 s i gm a l a ye r s u ni f or m lyd i s t r i b u t e d i n t h e u p p e r 1 0 0 m e t e r s , a n d 2 0 u n e v e n l y d i s t r i b -u t e d z l e v e l l a y e r s b e l o w 1 0 0 m . T h e m o d e l i s r u n u s i n g t h eM e l lo r - Ya m a da L e v el 2 t u r bu l e nc e c l o su r e s c he m e f o r v e r-t i c al m i x in g , q u as i - th i r d- o r de r u p w in d a d v ec t i on o f s c a la r sa n d m o m en t u m, a n d s e c o n d- o r de r c a l cu l a ti o n s o f t h e v e r ti c a l

a d ve c ti o n t e rm s , h o ri z on t al p r es s ur e g r ad i en t t e rm s , a n di n te rp o la t io n o f t h e C o ri o li s t e rm . I n it i al t e mp e ra t ur e a nd

Figure 1. L oc at io n o f t he w es te rn G ul f o f M ex ic o a nds it es o f i nt er es t. S ec ti on s w he re d et ai le d a na ly si s i s

presented are indicated with capital letters, and circlesi n d ic a t e t h e s i t es ( s t a ti o n s) o f s u r fa c e c u r r e n ts i n F i g ur e 1 1.

T id e g au ge l oc at io ns a re i nd ic at ed : G al ve st on ( GLV ),C i ud ad M a de r o ( C dM ) , Ve r ac r uz ( V ER ) , a nd P ro gr e so(PRG).

19 - 2 Z AVALA-HIDAL GO ET AL.: SEASONAL CIRCUL ATION WESTE RN SHELF


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s a l in i t y f i e l d s a r e d e r iv e d f r o m t h e 1 9 9 4 Wo r l d O c e a n A t l a s[National Oceanic and Atmospheric Administration, 1 9 94 ] .T h e m o de l i s f or c ed w i th c l im a to l og i ca l m on t hl y s u rf a cef l u x e s o f h e a t a n d m o m e n t u m d e r i v e d f r o m t h e C o m p r e h e n -s i v e O c e a n A t m o s p h e r e D a t a S e t [DaSilva et al., 1 9 9 4 ] . T h ed i sc h ar g e o f 3 0 m a jo r r i ve r s i s p r es c ri be d u si n g m o nt h ly

m e an s f o r t h o se i n t he U n it e d S t a te s a nd a n nu al m e an s f o r t h o s e i n M e x i c o . A s u r f a c e s a l i n i t y f l u x u n i f o r m i n s p a c e a n dt im e i s a ls o a pp li ed t o b al an ce t he r iv er in e i np ut o f f re shw a t er . C l i m a t ol o g ic a l v o l um e f l u x a t t h e o p en b o un d a ry i nt he C ar ib be an S ea i s c al cu la te d f ro m a m ea n d yn am ict o p og r a ph y r e l at i v e t o 1 0 00 m , d e r iv e d f r o m h i s to r i c h y d r o -graphic measurements [Fox et al., 2 0 0 1 ] , a n d t o a b a r o c l i n i cv e l oc i t y p r o fi l e d y na m i ca l l y c o ns i s te n t w i t h t h e c l i ma t o lo g -i c a l t e m pe r a tu r e a n d s a l in i t y f i e ld s .

[7] T he s i mu l at i on r e pr od uc e s w e ll t h e l a rg e -s c al e f e a-t ur es i n t he G oM , s uc h a s t he L oo p C ur re nt a nd t hea s s oc i a te d l a r ge a n t i cy c l on e s . T h e m e a n n o rt h e rn p e ne t r a-t io n o f t he L oo p C ur re nt i n t he m od el i s a pp ro xi ma te ly

2 6 . 5

N , c om pa re d t o 2 7

N f ro m t he m ea n dy na mi ct op og ra ph y r el at iv e t o 1 00 0 m [Fox et al., 2 00 1] , a nd t o

t h e a v e r a g e o f 2 7 . 5 N c a l c u l a t e d f r o m f i v e y e a r s o f s a t e l l i t ed a t a [Vukovich, 1 9 8 8 a , 1 9 8 8 b ] . T h e m e a n t r a n s p o r t b e t w e e nt he Yu ca ta n P en si ns ul a a nd C ub a i s 3 2 S v ( 1 S v =106 m s1) , w hi ch i s s om ew ha t l ar ge r t ha n p re vi ou sestimates [Gordon, 1 9 6 7 ;Roemmich, 1 9 8 1 ] a n d t h e r e c e n t l ym e as ur e d 2 3. 8 S v b y Sheinbaum et al. [ 2 00 2 ] . T h e s i m u-

l a ti on g e ne r at es a s i mi l ar c u rr e nt s t ru c tu r e i n t h e Yu ca t anC h a nn e l t o t h a t o b t a i ne d f r o m r e c e n t m e a s u r em e n t s [Ochoaet al., 2 00 1; Sheinbaum et al., 2 00 2] , w it h t he c or e o f t hec u rr e nt c on f in e d t o t he u p pe r 8 0 0 m o n t h e w e s te rn s i de o f t h e C ha n ne l , a c ou n te r cu r re n t i n t h e C ub a n s i de , a nd t w od e e p c o u n t e rc u r re n t s a l o ng t h e e a s te r n a n d w e s te r n s l o pe s[Morey et al., 2 0 03 a] . T he L oo p C u rr en t s he d s l a rg e a n ti -c yc lo ne s a t i rr eg ul ar i nt er va ls , v ar yi ng f ro m 2 .7 t o1 5 m o n t h s , w i t h a m e a n o f 9 . 9 m o n t h s , s i m i l a r t o t h e r e s u l t sr e p or t e d b ySturges and Leben [ 2 00 0 ] f o r a 3 0 -y e a r p e r i o d.I n t h e m o de l t he a v er a ge w e st w ar d t r aj e ct o ry o f t h e L oo pC ur re nt e dd ie s r ea ch es t he w es te rn G oM a ro un d 2 6 N,w h i ch i s s l i gh t l y n o r t h c o m pa r e d t o t h e o b se r v ed t r a je c t or y

t h at r e ac he s t h e w e st er n G oM a ro u nd 2 4. 5

N . T he m od e la l s o s i m u l a te s w e l l s m a l l e r -s c a le f e a tu r e s, s u c h a s t h e s m a ll

Figure 2. T ra n sp o r ts , f i l te r e d w i t h a 3 0 - da y r u n ni n g a v e r a ge , a c r os s t h e s e c t i o ns i n di c a te d i n F i gu r e 1 .T hi c k l i ne s i n di c at e t h e t r an sp o rt s b et w ee n t he c oa s t a nd t h e 2 5 m i s ob at h f r om s ur f ac e t o b ot t om . T hi nl i ne s i n di c at e t h e t r an s po rt s b et w ee n t h e 2 5 m a nd 5 0 m i s ob at h s. P o si t iv e v al u es i n di c at e u pc o as t transports.

Z AVA L A - H ID A L G O E T A L . : S E A S O NA L C I R CU L AT I O N W E S T E RN S H E L F 19 - 3


4/19

f r o nt a l e d d ie s a l o ng t h e L o op C u r re n t , t h e f r o nt a l i n s ta b il -i t i es a l o ng t h e w e s t e r n c o n t i n en t a l s h e l f , l a t er d i s cu s s ed i nt hi s p ap er, a nd s ma ll e dd ie s a nd f il am en ts o n t he We st F l o r i d a S h e l f . I n t h i s s t u d y , t h e l a s t s e v e n y e a r s o f a 1 0 - y e a r m o d el s i m ul a t io n a r e a n a ly z e d. D e t ai l e d i n f o r m at i o n o f t h e

NCOM and the GoM simulation are given byMartin [2000]and Morey et al. [2003a], respectively.

[8] S ev e ra l s o u rc es o f h y d ro gr a ph ic d at a a r e u s ed f o r t h i ss t ud y: h i st o ri c al d a ta f r om t he N a ti o na l C l im a ti c D a ta

C en te r, d at a f ro m s ev er al c ru is es i n t he M ex ic an s he l f k in dl y p ro vi de d b y L ui s S ot o f ro m t he I CM L- U NA M,

a n d d a t a f r o m Vidal et al. [ 1 9 8 8 , 1 9 9 0 , 1 9 9 4 b ] . T h e s a l i n i t yd at a f r om s om e c ru i se s r e qu ir e d c al i br at i on . F or t ho sec ru is es i n w hi ch t he re w er e d ee p c as ts , t he s al in it y w asa d ju s te d t o t h e c l im at o lo gy b y l i ne ar r eg r es s io n . I f t he r ew e re n o d e ep c a st s o r t h e s a l in it y d a ta h ad m a ny s pi k es , t hed a t a w e r e d i s ca r d ed .

[9] I n a dd it io n, m on th ly m ea n s ea l ev el d at a f ro m t id eg a ug es o bt a in e d f r om Gonzalez et al. [ 19 97 ] , s e a l e ve la t m os p h er i c p r e ss u r e d a ta f r o m t h e N C E P R e a n a l ys i s p r o -

v i de d b y t h e N O AA - CI R ES C l im a te D i ag n os t ic s C e nt e r,B o u l de r , C o l or a d o, U S A ( h t tp : / / ww w. c d c .n o a a. g o v ), a n d

Figure 3. M o nt h l y m e a n s u r fa c e c u r r en t s f r o m 7 y e a rs o f m o de l d a t a a l o n g t h e w e s t e rn s h e l f o f t h e G u l f o f M e x ic o . S h o wn a r e t h e i s ob at h s o f 2 5, 5 0, a n d 2 0 0 m .



5/19

monthly mean chlorophylla concentrations estimated fromt h e S ea Wi FS d at a a r e c on s id e re d . AV H RR i m ag e s w e re

processed at IG-UNAM.

3. Results

[10] O n a s e as on a l s c a le t he c i rc ul a ti o n o v e r t h e L AT EXs he lf i s w el l k no wn , b ut t he re a re c on si de ra bl y f ew er s tu di es s ou th o f t he R io G ra nd e m ou th . H er e, t he d is -c u ss i on f r eq ue nt l y r e fe rs t o t h e L AT EX s h el f i n o rd e r t o

r el at e i ts d yn am ic s t o t ha t o f t he M ex ic an s he lf a nd a s ar e fe r en c e t o c om p ar e m od e l r e su l ts w i th p r ev io u s w o rk s .

R es ul ts a re a ls o c om pa re d w it h a va il ab le d at a f ro m t heM e x ic a n s h e lf .

3.1. Along-Shelf Transports and Surface Currents

[11] Tr an sp or ts o n t he w es te rn s he lf o f t he G oM , c al cu -l a te d b y h o ri z on t al l y a n d v e rt i c al l y i n te g ra t i ng m o de lv el o ci t ie s, h av e a s tr o ng s ea so n al c om p on en t b ut w i thr e gi o na l d i ff er e nc e s. I n o r de r t o e va l ua t e t h ei r b e ha v io r a l on g t h e w e st e rn s he l f, t r an sp o rt s a re c om p ut e d a c ro s ss e ve r al m e ri d io na l o r z o na l s ec t io ns ( F ig u re 1 ). F r om t he

R i o G r a n d e t o t h e s o u t h e r n m o s t p a r t o f t h e B a y o f C a m p e c h et he m ai n t ra ns po rt s a re a s f ol lo ws : f ro m S ep te mb er t o

Figure 3. (continued)



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M a r ch , t h e t r a n s p or t s a c r os s s e c ti o n s b e t w e en t h e c o a s t a n dt h e 2 5 m i s o b a t h , a n d b e t w e e n t h e 2 5 m a n d 5 0 m i s o b a t h s ,a r e d ow n co a st ( Fi g ur e 2 ). F ro m M a y t o A u gu s t t he c i rc u-

l a ti o n r e ve rs e s a nd t r an sp or t s a r e u pc oa s t o ve r t he TAV Es h el f . T he p er i od o f t he u pc oa s t c i rc ul a ti on i s s ho r te r t ha nt h a t o f t h e d o w nc o a st c i r cu l a ti o n a n d r e d uc e s p r o gr e s si v e lyt o t h e n o r th a nd e a st a lo n g t h e L AT EX s he l f. A t s ec t io n C ,t h e u p co as t c i rc u la t io n p e ri o d r e du ce s t o l es s t h an t h re em o nt h s, f r om m i dd l e M ay t o e ar l y A u gu s t; o n t he L AT EXs h e l f a t s e c t i o n B , i t d e c r e a s e s t o l e s s t h a n t w o m o n t h s , f r o me ar ly J un e t o l at e J ul y, a nd i s n ea rl y l im it ed t o J ul y a t s e ct i on A . A l on g t h e w e st e rn C a mp ec h e B a nk t h e c i rc ul a -t i on i s u pc o as t t hr o ug h ou t t h e y e a r ; i n t h e m i d d le s he l f t het r a n s p o r t h a s a m a x i m u m i n J u l y t h a t d r o p s a l m o s t t o z e r o i nS e p t e m b e r a n d r e m a i n s w e a k u n t i l M a r c h , i n c r e a s i n g s l o w l yu n ti l J u ne . I n t he B a y o f C a mp ec h e, b et w ee n s e c t io ns I a nd J ,d u ri n g t he f a ll - wi n te r p er i od t h er e i s a c on f lu e nc e r e gi o n

produced by a downcoast coastal current on the TAVE shelfa n d a n u p c o a s t c u r r e n t o n t h e w e s t e r n C a m p e c h e B a n k . T h i sc o nf l ue n ce g en er a te s a n o ff s ho r e t r an sp o rt , w h ic h i s d i s-c us se d i n d et ai l l at er o n. I n t he s ou th er n Te xa s s he lf ,

between sections C and D, there is another confluencer e g io n w i t h i t s c o r re s p on d i ng o f fs h o re t r a ns p o rt , a l t h ou g hm o st o f t he t i me t he o f fs ho r e t r an s po rt r e ci r cu l at e s w i th i nt h e s h e l f . T h e c o n f l u e n c e r e g i o n s , t h e c u r r e n t r e v e r s a l o n t h eTAV E s h e lf , a n d t h e r e c ir c u la t i on o n t h e L AT E X s h e l f a r ec le ar ly i de nt i fi ab le i n t he 7 -y ea r m on th ly m ea n s ur f ac ec u r re n t s ( F i gu r e 3 ) .

[12] A l th ou gh t he t r an sp or t s i n t h e o u t er s h el f a r e i n f lu -e n c ed b y t h e e d d i e s , o n e m a j or c h a ra c t er i s ti c o f t h e L AT E X

s h e l f d u r i n g t h e f a l l , w i n t e r , a n d s p r i n g s e a s o n s i s t h a t , w h i l eo ve r t he i nn er s he lf t he re i s a w es tw ar d c ur re nt , o n t he

i n te rm e di a te a nd o ut e r s h el f t h e m e an c u rr e nt i s e as t wa r d.T hi s c h ar a ct e ri st i c c an b e o bs e rv ed i n t he t r an s po rt s b e-t w e e n t h e 2 5 m a n d 5 0 m i s o b a t h s a t t h e m e r i d i o n a l s e c t i o n

A , a nd i n t h e m e an s u rf a ce c ur r en ts . T he c ou n te r cu r re n t i sd r iv en b y t h e a nt i cy c lo n es t h at d ec a y n e x t t o t h e s l o pe a sw a s r e p o r t ed b yNowlin et al. [ 1 99 8 ]. T h e s h e a r w i t hi n t h es he lf i s a ls o o bs er ve d i n t he m ea n s ur fa ce c ur re nt s a t t hes o u t h e r n p a r t o f t h e T e x a s s h e l f a n d t h e n o r t h e r n T a m a u l i p a ss he lf ( Fi gu re 3 ). I t i s n ot ic ea bl e t ha t t he t ra ns it io n f ro md o wn c oa st t o u pc o as t i n t he n or t h TAV E s h el f f r eq ue n tl y

begins with the coexistence of downcoast currents in thei n n er s h el f a n d u p c o as t c u r re n t s i n t h e o u t e r s h e l f , f e w d a ysl a t er t h e d o w n c oa s t c u r re n t w e a ke n s a n d v a n is h . T h e i n n e r s h e lf p r e fe r e nc e o f t h e d o w n c o as t c u r re n t i s c o ns i s te n t w i t hi t s l o w er d e n si t y, b e c au s e o f t h e l o w er s a l in i t y.

[13] T he c u rr e nt r ev e rs a l d oe s n ot o cc u r s i mu lt a ne o us l ya l on g t h e e n ti r e w e st e rn s he l f. I t h a pp en s b e tw e en A ug u st

a n d S ep t em be r, b e gi n ni n g i n t he n or t h a nd p ro p ag a ti n gd o w nc o a st w i t h a p p r o xi m a te l y a 1 - m on t h l a g b e t we e n t h eL AT EX s h el f a n d t h e s ou t he r n B a y o f C a mp e ch e. O n t hew e st e rn C a mp ec h e B a nk , t he t r an sp or t s b e tw e en t he 2 5 ma n d 5 0 m i s o b a t h s h a v e a s t r o n g s e a s o n a l s i g n a l w i t h a p e a k i n J ul y A u gu st a nd a m in im um i n N ov em be r ( Fi gu re 2 ),a l t ho u gh t h e re i s n o t a l o w -f r e qu e nc y c u r re n t r e v er s a l o v e r t h e i n n e r s h el f . D u r in g t h e s p r in g t r a ns i t io n , f r o m M a r ch t oA pr il , t he r ev er sa l i n t he c ur re nt d ir ec ti on b eg in s i n t hes o ut h a n d m ov e s n or t hw a rd , a ls o w i th a bo u t a 1 - mo nt h l agf r om s ec t io n I a nd s e ct i on E ( F ig ur e 3 ) .

3.2. Confluence Regions

[14] T h e d o w n co a st c i r c ul a t io n o n t h e T AV E s h e l f d u r i n gt h e f a l l- w i nt e r s e a so n r e a ch e s t h e s o ut h e rn m o st p a r t o f t h e

Figure 4. M ea n s ur fa ce s al in it y f ro m t he m od el o ut pu t f or : ( a) M ay t o A ug us t a nd ( b) O ct ob er t oF e b r u a r y . V e c t o r s r e p r e s e n t t h e m a i n c u r r e n t s f o r t h e p e r i o d w i t h t h e s a m e s c a l e a s i n F i g u r e 3 . S h o w n a r et h e 2 0 0 m a n d 1 0 0 0 m i s o b a t h s .



7/19

B a y o f C a mp ec h e, b et w ee n s e ct i on s I a n d J , w h er e i t m e et sa n o p po s i ng c u r re n t . T h is c o nf l u en c e g e n er a t es a n o f f sh o r et r an sp or t o f a t l e as t 0 . 1 S v. A n i n di c at i on o f t he o ff s ho r et r an sp or t i s t h e s e as on a li t y o f t he l o w- s al i ni t y w a te r i n t hes ou th er n p ar t o f t he B ay o f C am pe ch e i n b ot h m od el a ndh i st o ri c al h yd r og r ap hi c d at a ( Fi g ur e s 4 a nd 5 ) . T hi s w a te r c o m es f r o m r e m o t e a n d l o c al r i v er s , m a i nl y t h e C o a t z a co a l -c o s a n d G r i ja l v a- U s um a c i nt a . O f f -s h e lf t r a ns p or t s a r e a l s o

n o t ic e a bl e f r o m h y dr o g ra p h ic d a t a w i t h l o w er - s al i n it y v a l -u es i n t he f al l- wi nt er t ha n i n t he s um me r, w he n t he r iv er

d i s ch a r ge s p e a k. T h e i m p a c t o f o f f -s h e lf t r a ns p o rt s m a y b eo f p a rt i cu la r s i gn if i ca n ce b ec a us e t h e s ou t he rn B ay o f C a m pe c h e i s a n i m p or t a nt f i s he r y a n d o i l e x t ra c t io n r e gi o n[Soto and Escobar, 1 99 5] .

[15] T h e o t h er c o n f l u en c e r e g i on i s o n t h e s o u t h er n T e xa ss h e l f , b e t w e e n s e c t i o n s C a n d D . T h i s c o n f l u e n c e i s s t r o n g e r i n A p ri l a n d M a y. O f fs h or e t r an sp o rt s i n t h is r e gi o n h av e

been reported previously [Muller-Karger et al., 1 9 9 1 ;Biggs

and Muller-Karger, 1 99 4; Walker et al., 1 99 6] , b ei ngi d en t if i ed b ec au s e o f t he r e la t iv e ly h ig h c hl o ro ph yl l a

Figure 5. I nd iv id ua l t em pe ra tu re a nd s al in it y o bs er va t io ns f ro m h is to ri ca l h yd ro gr ap hi c d at a.Te mp er at ur e f ro m M ay t o A ug us t ( a) a t 5 m d ep th a nd ( b) a t 3 0 m d ep th . S al in it y a t 1 0 m d ep th( c ) f r o m O c t o be r t o M a rc h a nd ( d ) f r o m M a y t o A u gu s t.



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c o nt e nt o f t he i n ne r s h el f w at e rs . T h e o ff s ho r e t r an s po r tss o me ti m es c ro s s t he s he l f b r ea k b e ca us e o f t he j et s a s so ci -a te d w it h t he e dg e o f a n e dd y o r a p ai r o f e d di es . M os t o f t h e p re v io u sl y r e po r te d o ff - sh el f t r an sp or t s w e re o f t h isc a te go r y b u t, f r om t h e 7 - ye a r m e an m o de l s i mu la t io n , i t i sf o u n d t h a t t h e o f f s h o r e t r a n s p o r t s h a v e a s e a s o n a l p r e f e r e n c et h a t m a y m o d u l a te t h e c r o ss - s he l f t r a ns p or t s f a v or e d b y t h e

presence of eddies.[ 16 ] T he c on ve rg en ce o f a lo ng -s he lf c ur re nt s o n t he

w e s te r n s h e lf i s c o m pe n s at e d b y o f f sh o r e t r a n s p or t s m a i nl yl o ca t ed i n t h e s o ut he r n B a y o f C a mp ec h e, w h er e o pp o si n gc u r r e n t s m e e t i n f a l l a n d w i n t e r , a n d i n t h e s o u t h w e s t o f t h eL AT E X s h e lf . T h e o f f sh o r e t r a n s po r t s a r e b a l an c ed b y t h ei n pu t o f w a te r f r om t he e as t er n B a nk o f C a mp e ch e, e a st o f 90W, a nd b y f lu xe s o nt o t he e as te rn L AT EX s he lf f ro mo f f sh o r e a n d t h e e a s t.

3.3. Temperature and Salinity Fronts

[17] D u ri n g t h e f a ll a n d w i n te r, l o w- s al i ni t y w a t er f r omt h e M i s si s s ip p i -A t c ha f a l ay a r i v er s y s te m i s a d v ec t e d o n t ot h e TAV E s h e lf , a l o ng t h e i n ne r L AT E X s h el f , d e v el o p in gs ha rp f ro nt s o n t he Ta ma ul ip as s he lf c lo se t o t he 5 0 mi s o ba t h . T h e f r o nt s a r e d e t e c t ed i n s e a s u r f a c e t e m p e r at u r es at el li te i ma ge s a nd a re f ou nd i n t he m od el r es ul ts e ve nt ho ug h t he m od el i s f or ce d w it h m on th ly m ea n f lu xe s

( F ig ur e 6 ) . T he a dv e ct e d w a te r c an b e a r ou nd 5

C c oo l er t h an t h e w a t e r o f f t h e s h e lf .

[18] D ur in g s um me r, w he n t he c ir cu la ti on o n t he Ta m-a u l ip a s s h el f r e v er s e s, t h e M i s si s s ip p i - At c h af a l ay a w a t e r d o e s n o t r e a ch t h e Ta m a ul i p as s h el f a n d r e m a i n s r e c i r c ul a t -i n g w i t h in t h e L AT EX s h el f , o r i s a d ve c te d e a st w ar d o f t heM is si ss ip pi R iv er m ou th ( se e F ig ur e 4 ). T hi s r es ul t i sc o n si s t en t w i t h p r e vi o u s s t u di e s o n t h e s e a so n a l c i r cu l at i o no f t h e L AT EX s he l f [ Cochrane and Kelly, 1 98 6 ; Li et al.,1996; Cho et al., 1 99 8 ;Nowlin et al., 1 9 98 ] a nd s t ud i es o nt h e f a t e o f t h e r i v er d i sc h ar g ed w a te r i n t h e n o r t he r n g u l f [Morey et al., 2 0 0 3 a , 2 0 0 3 b ] . T h e m a i n c o n t r i b u t i o n o f f r e s hw a t e r t o t h e T A V E s h e l f c o m e s f r o m t h e l o c a l r i v e r s , m o s t l yfrom the Grijalva-Usumacinta, Coatzacoalcos, Papaloapan,a n d P an u c o . T h es e r i v er s a r e r e l at i v el y s m a ll c o m pa r e d t ot he M is si ss ip pi a nd A tc ha fa la ya , s o, a lt ho ug h t he y d is -c h ar g e d ir e ct l y o nt o t h e TAV E a n d Ta ba s co s he l ve s a ndh a v e a m a x im u m d i s c h ar g e i n s u m me r , t h e a m o un t o f f r e s hw a te r o n t h e n o rt h er n TAV E s he l f i s l a rg er i n w i nt e r. T heR i o G r a nd e d i s ch a r ge i s i n s ig n i fi c a nt b e c au s e o f t h e F a l co na n d L a A m i s t a d d a m s t h a t s t o r e m o s t o f t h e d r a i n a g e o f t h eR i o G r a nd e b a s in .

3.4. Vertical Structure

[19] T he v er ti ca l s tr uc tu re o f t he w at er c ol um n o n t hew e st e rn s h el f o f t he G o M a l s o h as a s t ro n g s e a so na l it y. I nf a l l- w i nt e r, t h e TAV E s h e lf i s c h a ra c t er i z ed b y t h e f r e qu e n t

o c c ur r e nc e o f t e m pe r a tu r e i n ve r s io n s , w h i ch a r e s t a ti c a ll ys t a bl e b e c au s e o f t h e s u r fa c e l o w -s a l in i t y w a t e r. T h e i n v e r -

16 18 20 22 24 26 28

98W 97W 96W 95W 94W

21N

22N

23N

24N

25N

26N

27N

28N

29N

Figure 6. ( l e ft ) S e a s u r f a c e t e m p e ra t u re f r o m N O A A - AV H R R s a t e l l i te d a t a d u r i n g 1 D e c em b e r 1 9 9 6a nd ( r ig ht ) m o de l s e a s ur f ac e t em p er at u re f o r a p ar t ic ul a r d a y i n D e ce m be r. N ot e t h e p r es e nc e o f c o ldw a te r a l on g t h e w e s te r n s h e lf . T h e 2 0 0 m a nd 1 00 0 m i s ob a th s a r e s h o wn .



9/19

s i o n s o f t e m p e r a t u r e , w h i c h a r e e v i d e n t e v e n i n t h e s e a s o n a lm ea n, a re c au se d b y t he a dv ec ti on o f c ol d l ow -s al in it yw a te r f r om t h e L AT EX s he l f ( F ig ur e 7 ) . M o de l r e su l ts a res u p po r t ed b y h y d ro g r ap h i c d a t a f r o m d i f fe r e nt c r u is e s a n d

profiling floats that show the low surface salinity andt h e v er t ic al t e mp er a tu re i nv e rs i on s [Vidal et al., 1 98 8;Weatherly et al., 2 0 0 3] .

[ 20 ] D u ri n g t h e s u mm e r, t h e v er t ic a l s tr a ti f ic a ti o ni n c re a s es s i n ce t h e u p p e r o c e an b e co m e s w a r m e r a n d t h e rei s m o r e o f f s h or e p e ne t r at i o n o f t h e u p p e r l a y er l o w -s a l in i t y

w a te r, d i st i nc t iv e ly i n t h e r eg i on w i th s t ro ng e r r i ve r i n fl u-e n c e. M o de l r e s ul t s s u g g e st t h a t f r o m A p r il t o A u g us t t h e rei s u p w e l l i n g o n t h e T A V E s h e l f , w i t h a p e a k i n J u l y , a s s e e n

by the on-shelf penetration of intermediate depth water fromo f f o f t he s h el f ( F ig ur e 7 ). T he r e la t iv el y c ol d w a te r b ar e lyr e a ch e s t h e s u rf a c e, p r o ba b l y b e c a u se o f t h e s t r on g s t a bi l i tyo f t h e w a r m f r e s h u p p e r l a y e r . T h i s r e s u l t i s c o n s i s t e n t w i t ht h e u pw e ll i ng f o r t h e s o ut h er n L AT EX s h el f r e po rt e d b y

Nowlin et al. [ 19 98 ] a nd w i th h yd ro gr ap hi c d at a f r oms ev er al c ru is es a s c an b e o bs er ve d i n t em pe ra tu re m ap sf r om h i st o ri c al d a ta ( F ig ur e s 5 a a n d 5 b ).

3.5. Sea Level

[21] The sea l eve l is t he onl y v ari abl e t hat ha s be enm e as ur e d s ys t em a ti c al l y i n t he w e st e rn G o M. F ou r t i de

g a ug e s t at i on s r e pr e se n ta t iv e o f t h e w e st e rn s h el f o f t heG o M a r e a n a ly z e d: G a l ve s t on , C d . M a d er o , Ve r a cr u z, a n dP r og r es o ( Fi g ur e 8 a ). A c ha r ac t er i st i c o f t he a n nu al v a ri -a bi li ty o f t he s ea l ev el i s i ts s im il ar it y w it hi n a r eg io n o f m o r e t h a n o n e t h ou s a nd k i l om e t er s a l o ng t h e c o a s t . A l l t h es t a ti o n s h a v e a m a x im u m s e a l e v el i n S e pt e m be r O c t ob e r.I t p e ak s i n G a l v e s to n d u r in g S e pt e m be r a n d l e a d s t h e o t h er t hr ee l oc at io ns b y o ne m on th . T hi s m ax im um i s a ls oo bs er ve d i n t he m od el s ea s ur fa ce h ei gh t ( SS H) r es ul tsa n d i n t i de g a ug e o b se r va t io ns , b ut t h e m od e l h a s 1 - mo n th

l a g i n P r o gr e s o ( F i g u re 8 b ).[22] Ti de g au ge d at a s ho w a s ec on d s ea l ev el m ax im um

d u ri n g M a y, w h ic h i s m or e i n te n se i n G a lv e st o n t h a n i n t heo t he r t hr e e l oc a ti o ns . T hi s s e co n da r y m a xi m um i s w e llr e p r o d u c e d b y t h e m o d e l f o r G a l v e s t o n b u t n o t f o r t h e o t h e r locations, showing smaller amplitudes. Between the maximat h e r e a r e t w o m i n i m a , o n e i n J u l y a n d t h e o t h e r i n J a n u a r y F e b r u a r y t h a t h a v e s m a l l e r a m p l i t u d e a n d a r e l e s s c o n s i s t e n t a m o ng t h e s t a t i o ns t h an t h e S e p t e m be r O c t ob e r m a x i m u m( F ig ur e 8 b) . T he P ro gr e so s ea l ev e l c l im a to l og y h as a s l ig ht l yd i f f e r e n t p a t t e r n t h a n t h e d a t a f r o m t h e o t h e r t h r e e t i d e g a u g es ta ti on s; i t h as t he O ct ob er m ax im um a nd t he r el at iv em a xi m um i n M a y, b ut t h e m i n im a a r e r e a ch e d i n M ar c h

A p ri l a n d J u ne . T he m o de l s e a l e ve l s ig n al r e pr od u ce sbetween 30% and 76% of the observed variance (Figure 9).

Figure 7. M on t hl y m e an v er t ic al s t ru ct u re f r om m o de l o ut p ut i n s ec t io n E o f F ig u re 1 , a v er a ge d f r om( t op ) O c to be r t o F e br ua r y a nd ( b ot t om ) M a y t o A ug u st o f 1 y ea r.



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Figure 8. ( a ) L o c a ti o n o f f o ur t i d e g a u ge s t a t i o ns , ( b ) m o nt h l y m e a n s e a s u r f a ce h e i gh t f r o m t i d e g a u ges t a ti o ns ( t r ia n g le s ) a n d f r o m t h e m o d e l ( c i rc l e s) , ( c ) m o n th l y m e a n s e a l e v el a t m os p h er i c p r e ss u r e, a n d( d) m on th ly m ea n s ea s ur fa ce h ei gh t f ro m t id e g au ge s ( tr ia ng le s) a nd f ro m m od el w it h t he i nv er se

barometer effect (circles). Standard deviations of model data are less than 1 cm.



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[23] T he s i mu l at e d S S H f r om t he m od e l d o es n ot i nc l ud et h e i nv e rs e b ar o me te r c on t ri b ut i on d ue t o c ha ng e s o f t he

a t mo sp h er i c s e a l e ve l p re s su r e a t t he s ea s on a l f r eq u en cy.T he s ea l ev el p re ss ur e h as a c le ar s ea so na l p at te rn i n t her e g i o n , w i t h a n a b s o l u t e m a x i m u m i n D e c e m b e r J a n u a r y , as e co n da r y m a xi m um i n J ul y, a n d m i ni m um v al u es i n M a ya nd S ep te mb er. O n t he b as is o f t he N CE P- Re an al ys is

product, the pressure difference between the highest (Janu-a ry ) a nd l ow es t ( Ma y) v al ue s i s a ro un d 6 h Pa f or t he f ou r l o c at i o ns , a n d b e t w e e n t h e s e c on d a ry m a x im u m i n J u l y a n dt h e a b s o l u t e m i n i m u m i n M a y i s a r o u n d 4 h P a . T h e p r e s s u r ed i ff er e nc e b et w ee n J ul y a nd S ep t em b er i s s m al l er i n G a l-v es to n, C d. M ad er o, a nd Ve ra cr uz t ha n i n P ro gr es o b ya r ou n d 2 h Pa ( Fi g ur e 8 c) . T he l ow - fr e qu en c y v ar i ab il i tyo f t he s e a l e v el a t mo sp he r ic p r es s ur e d oe s n ot h a ve a d i re c t i m p a c t o n t h e d y n a m i c s o f t h e o c e a n b u t a f f e c t s t h e s e a l e v e l

a t a r a t e c l o s e t o 1 c m p er e ac h h Pa . T he a dd it io n o f t hei n v er s e b a r om e t er c o nt r i bu t i on t o t h e m o d el s e a l e v el y i e ld sa b et t er a gr e em e nt w i th t h e o bs er v ed s e a l e ve l ( Fi g ur e 8 d)a nd i nc re as es t he p er ce nt ag e o f t he s ea l ev el v ar ia nc ee xp la in ed b y t he m od el o ut pu t t o a r an ge b et we en 7 0%a n d 8 0 % f o r G a l v e s to n , V e ra c r uz a n d P r o g r e so , a n d t o 5 0 %f or C d. M ad er o ( Fi gu re 9 ). T he s ea l ev el f ro m t he m od elw i t h t h e i n v er s e b a r om e t er c o n tr i b ut i o n r e p ro d uc e s w e l l t h eS e pt e mb e r O c t o be r m a xi m um a n d t h e J ul y m i ni m um b ut w i th s m al l er a m pl i tu d e i n b ot h c a se s ( F ig ur e 8 d) . M o de lr e s u l t s l e a d t h e o b s e r v e d J u l y m i n i m u m i n V e r a c r u z a n d C d .M a d e r o b y o n e m o n t h . T h e m o d e l d o e s n o t a g r e e w e l l w i t ho b se r va t io n s f o r t he f i rs t f ou r m o nt h s o f t h e y ea r, w h en t he

o bs er ve d s ea l ev el s ig na l i s w ea k, e xc ep t a t G al ve st on .O t h er c o nt r i bu t i on s t o t h e l o w -f r e qu e nc y s e a l e v el v a r ia b i l-

i t y a r e n o t c o n s i d e r e d , s u c h a s t h e s t e r i c a n o m a l y , w h i c h i s abasin-scale contribution with amplitude of around 2 cm

[Greatbatch, 1 99 4; Mellor and Ezer, 1 99 5 ] , a nd t hea st ro no mi ca l a nn ua l t id e c om po ne nt , t ha t h as a s im il ar contribution.

4. Discussion

[24] Cochrane and Kelly [1986], Cho et al. [ 1 9 98 ] , a n dNowlin et al. [ 1 99 8] s h ow e d t h at , f o r m o st o f t h e L AT EX

s h el f , t h er e i s a h i gh c or r el a ti o n b e tw e en t he a l on g- c oa s t w i nd s t re s s c o mp on en t ( A CW S C) a nd t h e l o ca l c u rr e nt s .T he y i de nt if ie d a r eg io n o f c on ve rg en ce o f t he A CW SCa l on g t h e s o ut he r n c oa s t o f Te x as a n d a d i ve r ge nc e n ea r 93W.Cho et al. [ 1 9 9 8 ] c o n f i r m e d t h a t t h e m a i n c i r c u l a t i o no v er t he L AT E X s he l f i s w i nd d r iv en .

[25] T he A CW SC i s t he d om in an t f or ci ng o f t he c oa st alc ir cu la ti on w he n i t h as a r el at iv el y s ma ll s pa ti al a ndt e m p o r a l v a r i a t i o n ; t h a t i s , t h e t i m e s c a l e o f i n t e r e s t i s g r e a t e r t ha n t he i ne rt ia l p er io d a nd t he s ca le s o f t he a lo ng s he lf c ur re nt s a nd t he w in d f ie ld a re l ar ge r t ha n t he w id th o f t he s he lf . O ve r t he w es te rn s he lf o f t he g ul f, t he s ca le o f t h e a l on g s h el f c ur r en ts i s r e la t iv el y l a rg e c o mp a re d t o t hew i d t h o f t h e s h e l f , a r o u n d 5 0 0 t o 8 0 0 k m c o m p a r e d t o 5 0 t o1 0 0 k m r e sp ec t iv el y ; t ha t i s , t he d i st a nc e w h er e t h e a lo n g-s h el f c ur r en ts a r e h ig h ly c or r el at e d. I t i s a l so r e qu i re d t ha t t he R os sby n um be r ( R = V /f L) be sm al l; t hat is , t hen o nl i ne ar t e rm s a r e r e la t iv e ly s m al l c o mp a re d t o t he C or i -o l i s f o r c e t e r m s . O v e r t h e w e s t e r n s h e l f o f t h e G o M c u r r e n t s

are 0 . 5 m s1

s o , c o ns i d er i n g L 1 0 0 k m a n d F = 6 105 s1, R 0 .0 8, b ut i n t he c on fl ue nc e r eg io ns L i s

Figure 9. P e r ce n t ag e o f t h e m o n t hl y m e a n s e a l e v e l v a r i a n ce e x p la i n ed b y t h e m o d el ( w h it e b a r s) a n dby the model with the inverse barometer contribution (black bars) for Galveston (Gal), Cd. Madero( C d M) , Ve r a cr u z ( Ve r ) , a n d P r og r e so ( P rg ) .



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s m al l er a nd R c ou l d b e l ar ge e n ou gh t o m a ke t h e n o n li ne a r terms important.

[26] T he A C WS C i s t h e p ri n ci p al f or c in g m e c ha n is m o f t h e w e st e rn s h el f o f t h e G o M , i t e xp l ai n s t he m a in f ea t ur e so b se r ve d i n t he c i rc u la t io n. A l on g t he M ex i ca n s t at es o f Ta m a ul i p as a n d Ve r a cr u z , b e t we e n 1 8 . 5N and 26N, the

A C WS C i s u pc o as t f r om A pr i l t o A ug u st a nd d ow n co a st f ro m S ep te mb er t o M ar ch ( Fi gu re 1 0) . I n t he B ay o f C am pe ch e, e as t o f 9 5. 5W, t he A CW SC h as a n u pc oa st d ir ec ti on d ur in g t he w ho le y ea r a nd i s m or e i nt en se i nw i n te r. T h e A C W SC h a s a s m a ll - s ca l e v a r ia t i on a s s oc i a te dw i th t h e c oa s tl i ne v a ri a ti on s a l on g t h e w e st e rn C a mp e ch eB a n k . I n t h e L A T E X s h e l f t h e u p c o a s t A C W S C r e a c h e s t h ew es te rn s he lf i n J ul y a nd i s v er y w ea k i n t he e as te rn a ndc en tr al s he lf . T he r ev er sa l p er io d i n t he DaSilva et al.[ 1 99 4] c l im a to lo g y i s l i mi t ed t o J ul y, w h ic h i s d if f er e nt f ro m t he o bs er va ti on s f or t he s um me rs o f 1 99 2 t o 1 99 4w he n t he u pc oa st A CW SC l as t f or t wo m on th s a nd w ass t r on g er [Nowlin et al., 1 9 98 ] .

[27] T he d i r e ct i on o f t h e w i n ds i n t h e G o M i s d e t er m in edby the seasonal position of the high-pressure systems: in the

f a l l a n d w i n te r h i gh - p re s s ur e s y st e m s m o v e f r o m t h e n o r th -w es t c on ti ne nt al U ni t ed S ta te s i nt o t he g ul f g en er at i ngn or th ea st er ly w in ds i n t he w es te rn g ul f, w hi le i n t hes u m me r t h e i n t en s i f ic a t io n a n d w e s tw a r d d i s pl a c em e n t o f t he B er mu da H ig h a nd t he w ar mi ng o f t he c on ti ne nt alU n i te d S t a te s g e ne r a te s o ut h e as t e rl y w i n ds . T h e m e r id i o na l

c o m p o n e n t i s i n t e n s i f i e d i n t h e w e s t e r n g u l f b y t h e b l o c k i n ge f f ec t o f t h e S i e rr a M a d re O r i en t a l m o u n t ai n r a n ge t h a t h a sa n e l e va t i on o f a r o un d 2 0 00 m . T h e d e v e l op m e nt a l o ng t h ey e ar o f t he s e h ig h- p re ss u re s y st e ms r e su l ts i n a b im o da le vo lu ti on o f t he s ea l ev el p re ss ur e o ve r t he G oM , w it hm a xi m a i n w i nt e r a n d J ul y ( F ig u re 8 c ).

[28] T he s t ro ng es t a n d m o s t v a ri a bl e c oa s ta l c ur r en ts , a t t he s ea so na l f re qu en cy, o f t he w es te rn s he lf a re o n t heT A V E s h e l f , r e a c h i n g a m a g n i t u d e o f m o r e t h a n 0 . 7 0 m s1.A t t h e s e as on a l f r eq u en c y, t h e w i nd s i n t h is r eg i on a r e a l som o re i nt e ns e a nd v a ri a bl e t ha n o ve r t he r e st o f t h e w e st e rns h el f . T he c o rr e la ti o n b e tw e en t h e A C WS C a n d t he 7 - ye a r m od el m on th ly m ea n a lo ng s he lf c ur re nt s i s l ar ge r t ha n

0 . 9 0, s u gg e s ti n g t h a t t h i s i s t h e m a i n f o r ci n g m e c h a ni s m o f t h e l o w -f r e qu e n cy s h el f c u r re n t s i n t h i s r e g i o n ( F i gu r e 1 1) .

Figure 10. M o n th l y m e a n s o f t h e a l o n g- c o as t w i n d s t r e ss c o m po n e nt c o m pu t e d o v e r a s m o ot h e d 2 5 mi s o ba t h . P o si t i ve v a l ue s i n di c a te d o w nc o a st d i r ec t i on .



13/19

I n t h e s o u t h e r n p a r t o f t h e B a y o f C a m p e c h e , a t 9 3 . 0W, thec o rr e la t io n d e cr e as e s s i gn i fi c an t ly b e ca us e o f t h e h i gh

v a ri a bi l it y i n s p ac e o f t h e A C W S C ( se e F i gu re 1 0) . I n t h at r e gi o n, o ff s ho r e t r an sp or t s a r e i m po rt a nt d ur i ng f a ll a n d

w i n t e r b e c a u s e o f t h e c o n f l u e n c e o f c u r r e n t s , t h e a l o n g s h e l f c ur re nt s v ar y o ve r r el at i ve l y s ho rt s ca le s, t he r ef or e t he

R o ss by n um b er m a y n o t b e s m al l a n d t h e n on l in e ar t e rm ss i g ni f i ca n t . T h e v a ri a b il i t y o f t h e m o n th l y m e a n c u r re n ts o n

Figure 11. M o n th l y m e a n s u r f ac e c u r r e nt s f r o m 7 y e a rs o f m o d el d a t a a t l o c a t io n s i n d i ca t e d i n F i g u r e 1( c ir c le s) a n d m on t hl y m e an o f t h e a l on g -c o as t w i nd s t re s s c o mp o ne nt a t t h e c or r es p on di n g s ec t io n( p l u s e s ) . T h e v a l u e s o f t h e c o r r e l a t i o n c o e f f i c i e n t s a r e a l s o i n d i c a t e d i n e a c h p l o t . E r r o r b a r s r e p r e s e n t o n es t a nd a r d d e vi a t io n . P o si t i ve v a l ue s i n d ic a t e d o w nc o a st c u r re n t s a n d w i n d s t r es s .



14/19

t h e w e st e rn C a mp ec he B a nk ( Fi g ur e 3 ) , m e as u re d a s t hed i ff er e nc e b et w ee n t h e m a xi m um a nd m i ni m um v al u es , i sl es s t ha n 0 . 10 m s1, w h il e i n t he TAV E s he l f i t i s g r ea t er t h a n 1 . 0 0 m s1. T he v a ri a bi l it y o f t h e w i n d f o rc in g i s a l sos m a l l e r o v e r t h e w e s t e r n C a m p e c h e B a n k t h a n i n o t h e r a r e a so f t he w e st e rn G o M. T he r ef o re t h e c or r el a ti o n c o ef fi c ie nt

between the ACWSC and the surface currents is meaning-

l e ss . T he m o nt h ly m e an t r an sp or t s a nd s ur f ac e c ur r en t s o nt h e w e s t e r n C a m p e c h e B a n k s u g g e s t t h a t t h e m a i n f o r c i n g i st h e l o c a l A C W S C , w h i c h i s v e r y s t a b l e t h r o u g h o u t t h e y e a r ,a nd t ha t t he r el at iv el y s ma l l v ar ia bi li ty i s p ro du ce d b yr em ot e f or ci ng d ue t o l on g w av el en gt h c oa st al w av esa s s oc i a te d w i t h t h e c h a ng e s i n t h e c i r cu l a ti o n o n t h e TAV Eand LATEX shelves.

[29] T h e s t an d a rd d e v i a ti o n s o f t h e m o n th l y m e a n s u rf a c ec u r r e n t s i s v e r y i n f o r m a t i v e a b o u t h o w t h e w i n d f o r c i n g a n dt h e e d d i e s a f f e c t e a c h r e g i o n . T h e l o c a t i o n o f t h e c o r e o f t h ec o as t al c ur r en t f or e a ch s ec t io n i s s ub j ec t iv el y c ho se n a n dr e fe r re d f r om h er e o n a s s t at i on s w i th t h e c or r es p on di n gl et te r ( Fi gu re s 1 a nd 3 ). T he s ta ti on s a re l oc at ed n ea r t he

c o a st , b e t we e n 1 5 a n d 3 0 k m , b u t a t d i f f e r en t d e p t h s. W h e nt h e s h e lf i s w i de t h e s t a ti o ns a r e i n s h al l ow w a te r, n ea r t he2 0 m i so ba th , a s o n t he L AT EX s he lf a nd t he C am pe ch eB a n k . W h e n t h e s h e l f i s n a r r o w , t h e c o r e o f t h e c u r r e n t a n dt h e c o r re s po n d in g s t a ti o n , a l t h o u gh c l o se r t o t h e c o a st l i n e,a r e i n d e ep er w a te r, a s i s t h e c a s e o n t h e T AV E s he l f.

[30] S in c e t h e m o de l i s f o rc e d w i t h c l i m at ol o gi c al w i nd s ,a l l t h e i n te r an n ua l v ar i ab i li t y i n t h e a l on g s h el f c ur r en ts i se x pe c te d t o b e a s so ci a te d w i th t he m e so s ca l e e d di e s t h at i n te r ac t w i th t he s l op e . T he e dd i es d o n ot h av e a s e as o na l

behavior, although they have preferred paths. Therefore, ase x pe c te d , t h e s e ct i on s l o ca t ed w h er e t he s he l f i s w i de h a ves m a l l e r v a r i a n c e b e c a u s e o f t h e s m a l l o r n e g l i g i b l e i n f l u e n c eo f t h e e d d i e s o v e r t h e i n n e r s h e l f . A t s t a t i o n s K a n d I a t t h eC am pe ch e B an k t he v ar ia nc e i s v er y s ma ll , a ls o i n t heL A T E X s h e l f i n s t a t i o n s A a n d B . I t i s n o t i c e a b l e t h a t d u r i n gl a t e s p r i n g a n d s u m me r t h e v a r i a n ce i s l a r ge r. T h is s u gg e s tst h at t h e e d d ie s h av e m o re i n fl u en ce i n t h is r e gi on w h en t heA C W S C i s v e r y w e a k . O n t h e T A V E s h e l f a t s t a t i o n s D t o Ht h e v ar i an ce i s l a rg er b e ca us e t h e s h e l f i s n ar r ow e r b ut t hes i g n a l a n d t h e c o r r e l a t i o n s o f t h e c u r r e n t s w i t h t h e A C W S Ca r e l ar ge r. T he v al ue o f t he c or r el at io n c oe ff ic ie nt i ss t at i st i ca ll y s i gn if i ca nt f o r t he 9 5% c on f id e nc e l e ve l f o r 1 2 p o i n t s ( 1 0 d e g r e e s o f f r e e d o m ) , w h e n t h e c o r r e l a t i o n s a r el ar ge r t ha n 0 .5 8 a nd i s s ig ni fi ca nt f or t he 9 9% l ev el f or c o rr e la t io ns l a rg e r t h an 0. 7 1. S t at i on s E t o I i n t h e TAV Es h e lf h a ve c o r re l a ti o n c o e ff i c ie n t s l a r ge r t h a n 0 . 9 0 s u p po r t -

i n g t h a t t h e A C WS C i s t h e m a i n f o r c in g.[31] T he se r es ul ts a re c on si st en t w it h t he n ea r s ur fa ce

d r i f t e r s f o r s u m m e r a n d w i n t e r f r o m 1 9 8 9 t o 1 9 9 9 t h a t s h o wd ow nc oa st c ur re nt s i n w i nt er a nd u pc oa st c ur re nt s i ns u mm e r [Nowlin et al., 2 00 1, F ig ur es 5 .2 4 a nd 5 .2 5 ].T he U ni ve rs it y o f C ol or ad o G oM m od el r es ul ts a re a ls oc o n si s t en t w i t h t h e s e r e s ul t s ; t h e s t a nd a r d d e v i a ti o n o n t h eTAV E s he l f f or t h ei r m o de l i s v er y l a rg e a nd m a in ly a l on gc o as t , s u gg es t in g s t ro ng a l on g s he l f c ur r en ts w i th l a rg ev a ri a bi l it y, w hi c h a gr e es w i th o ur r e su l ts [Nowlin et al.,2 0 01 , F i gu r es . 5 . 3 1 a n d 5 . 3 2 ] . T h e r e su l ts o f t h i s s tu d yf o r t h e L AT EX s he l f r e gi o n a gr e e w i th t h e s e as o na l c i rc u-l a ti o n r e po r te d i n p re v io us s t ud i es [Cochrane and Kelly,1986; Cho et al., 1 9 9 8 , Nowlin et al., 1 9 98 ] t h at f ou nd t h at t h e m a i n f o r c i n g i n t h e L A T E X s h e l f i s t h e l o c a l w i n d s t r e s s .

T he c o rr e la t io n c oe f fi c ie nt s b et w ee n t h e s ur f ac e c u rr e nt sa nd t he A CW SC a re 0 .7 0 a nd 0 .5 5 a t s ta ti on s B a nd A ,r e sp ec t iv e ly. T he r e i s a c yc l on i c c i rc ul a ti on w i th a d ow n -c o a st c o a st a l c u r re n t d u r i n g t h e n o ns u m me r m o n th s a n d a nu p co as t c u rr e nt i n J u ly. T he p er i od o f t he u pc oa s t c i rc ul a -t i on s ho r te ns t ow a rd t he e a st ( F ig u re s 3 a nd 1 0 ). A c a re fu la n a l y s i s s h o w t h a t t h e s u m m e r A C W S C f r o m t h e DaSilva et

al. [ 1 99 4] c l im a to lo g ic a l w i nd i s w e ak e r a nd s ho r te r t h ant h os e o bs er v ed b et w ee n 1 9 92 a n d 1 99 4 b y Nowlin et al.[ 19 98 ]. T hi s c au se s t he p er io d o f u pc oa st c ur re nt i n t heL AT EX s he l f i n m o de l r e su l ts t o b e r e st r ic t ed t o a r ou nd 1 5d a y s p e r y e a r, d u r in g J u l y, c o m pa r e d w i t h o b s er v a ti o n s t h a t r e po r te d a r e ve r sa l o f 2 m on t hs [Nowlin et al., 1 9 98 ]. T her e l at i v el y s m a ll d i f fe r en c e i n w i n d d i r e c ti o n i s c r i ti c a l i n t h ed ev el o pm en t o f t he u pc oa st c u rr en t, b ei ng s h or t er a ndw e a ke r i n t h e s i m ul a t i on ( F i gu r e 3 ) t h a n i n o b se r v at i o ns .

[32] F ro m t he a na ly si s o f t he A CW SC a m ai n r es ul t i st h at t he w e st e rn s h el f , a s a w ho l e, i s a c o nv er ge n ce r eg i on .T h i s i s b e c a u s e f o r m o s t o f t h e y e a r t h e A C W S C h a s l a r g e r v al ue s i n t he n or th t ha n i n t he s ou th , w it h t he g ra di en ts

being more intense during fall and winter (Figure 10). Ac o ns eq u en ce i s t ha t t h er e i s a n et e x po rt o f w a te r f r om t hew e st e rn s h el f i nt o t h e d e e p o c ea n. T he m a ss i s b al a nc e d b yt h e i n fl u x f r om t h e e a st e rn B a nk o f C a mp e ch e, e nt r ai ni n gw at er f ro m t he Yu ca ta n C ur re nt , a nd f ro m t he e as te rnL AT E X s h el f . T h e r i v er s d i s ch a r ge h a s a s m a ll c o n tr i b ut i o nt o t he v ol um e b al an ce . T he t ra ns po rt s i n t he i nn er a ndi n t er m e di a t e s h e l f a r e a r o un d 0 . 1 t o 0 . 2 S v, t h e M i s si s s ip p ia n d A t c h a f al a y a R i v e r s h a ve a n a v e ra g e d i s ch a r ge o f 0 . 0 20S v w it h a p ea k i n A pr il o f 0 .0 35 S v a n d a m in im um i nS ep te mb er. O th er r iv er s h av e a s ma ll er c on tr ib ut io n. Ar e g io n w i t h s t r on g c o n ve r ge n c e i s t h e s o u th e r n T e x as s h el f ,

between sections C and D, particular ly from April toAugust. Cochrane and Kelly [ 1 9 8 6] i d e nt i f ie d t h i s c o n ve r -g en ce r eg io n, a nd a l at er s tu dy b as ed o n a n o pt im al lyi nt er po la te d w in d f ie ld d er iv ed f ro m b uo y d at a m ad e ad e ta i le d d es c ri p ti on o f t h e A C WS C a nd t h e c o nv er g en ce[Nowlin et al., 1 99 8] , a nd o ur r es ul ts c or ro bo ra te t hi ss h o wi n g t h e l i n k w i t h t h e T AV E s h e lf w i n ds . I n t h i s r e g i on ,e v e nt s o f o f fs h o re c u r re n t s c a n b e i d e nt i f ie d t h r ou g h r e m o t es e ns i ng s i nc e t h e e nt r ai ne d c oa s ta l w a te r i s c ol d w i th h i ghc h lo r op h yl l c o nc e nt r at i on . P r ev i ou s s t ud i es h a ve s h ow nc ro ss - sh el f t ra ns po rt s i n t hi s r eg io n a ss oc ia te d w it h t hei nt er ac ti on o f e dd ie s w it h t he s lo pe [Biggs and Muller-

Karger, 1 9 9 4 ; Muller-Karger et al., 1 9 91 ] , b u t t h e s e r e s u l t ss ho w t h at t h e c hl o ro ph yl l a c o nt e nt o f t h e c r os s- s he l f f il am en ts m ay b e m od ul at ed b y t he s ea so na li ty o f t he

c on ve rg en ce o f t he A CW SC , b ei ng l ar ge r f ro m A pr il t oA u gu s t. T he s e as o na li t y o f t h e m on t hl y m e an u pp er o c ea nchlorophyll a c o nt e nt f or t h e p e r io d 1 99 7 t o 2 00 1 f r om t heS e aW iF S i s c on s is t en t w i th t h es e r e su l ts ( F ig ur e 1 2) . T hel o c at i o n o f t h e e d d i e s m a y d e t er m i ne i f t h e o f f s h or e c u r re n t c r os s es t h e s he l f b r ea k o r t u rn s c y cl o ni c al l y o nt o t he o ut e r s he lf . A s ec on d r eg io n o f s tr on g c on ve rg en ce i s i n t hes o ut h er nm o st p a rt o f t he B a y o f C a mp e ch e, b e tw e en s e c-t i on s I a nd J , m o st e v id e nt f r om S e pt e mb e r t o F eb r ua ry. I nt h is r eg i on , m o de l r e su l ts s h ow s t ro ng c r os s- s he l f t r an s -

ports, which are evident because of the lower salinity of thes he lf w at er. A m ax im um i n t he c hl or op hy ll a contentd e te c te d t hr o ug h S ea Wi FS d at a d ur i ng t h e f al l a n d w i nt e r s u p po r t s m o d e l r e s ul t s t h a t s u g g e st t h a t t h e o f f sh o r e t r a n s -

port is more intense during this period (Figure 12). The high



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content of chlorophylla d u r i n g f a l l a n d w i n t e r i n t h e B a y o f C am pe ch e d oe s n ot s ee m t o b e p ro du ce d b y c oa st al u p-w e ll i ng d ue t o E km a n t r an sp or t s in c e t h e n or t he r ly w i nt e r w i n d s a r e d o w n w e l l i n g f a v o r a b l e f o r t h e w e s t e r n s i d e o f t h eB a y a nd n e ut r al f o r t he s ou t he r n. A l th o ug h t he w i nd s a reu pw el li ng f av or ab le f or t he e as te rn s id e o f t he B ay o f C a mp e ch e , t h ey a r e n o t s i gn i fi c an t ly m o re f a vo r ab l e t ha ni n t h e s u m m e r. H o w ev e r, t h e re m a y b e a s m a ll c o nt r i bu t i ont o t he i nc re as e o f n ut ri en ts i n f al l- wi nt er b ec au se o f t hei n cr e as e o f e n tr a in m en t i nt o t h e m i xe d l a ye r a s so ci a te d t o

t h e s e a s o n a l v a r i a t i o n o f t h e m i x e d l a y e r d e p t h , b u t t h i s t e r mi s s i mi l ar i n o t h e r r e g io n s o f t h e G o M .

[33] T he d is tr ib ut io n o f l ow -s al in it y w at er a nd t he s eas u r fa c e t e m pe r a tu r e a r e a l s o a f f e c t ed b y t h e s e a so n al c i r cu -l a ti o n. I n t h e n on su m me r m on t hs t h e l o w- s al i ni t y w a te r f r o m t h e M i s si s s ip p i -A t c ha f a l ay a R i v er s i s a d v ec t e d a l o ngt h e L AT E X s h e lf r e a ch i ng t h e TAV E s h e lf a n d t h e s o u th e r nB ay o f C am pe ch e, w he re as f ro m A pr il t o S ep te mb er t heM i s si s s ip p i -A t c ha f a la y a R i v er s l o w -s a l in i t y w a t er d o es n o t r ea ch t he TAV E s he lf . T he w ea ke r a nd s ho rt er u pc oa st c u rr e nt a l on g t h e L AT EX s he l f a l lo ws a m o re w e st w ar d

penetra tion of the low-sal inity water in model resultsc o m pa r e d t o o b se r v at i o ns ( F i gu r e 1 3 ). T h e f a l l s e a s u r f a c e

s a l in i t y d i s t r i bu t i on i s v e r y s i m i l ar t o o b se r v at i o n s b u t w i t ht h e l ow - sa l in i ty w a te r s l ig ht l y m o re c oa s ta ll y a t ta ch ed ,

w h ic h i s e xp e ct e d b e ca us e t he v ar i ab il i ty o f t h e s y no pt i c-s c a le w i n ds , w h i ch i n c re a s es t h e m i x in g o f t h e l o w -s a l in i t yw a te r [Garvin, 2 00 1] , i s n ot i n cl u de d i n t h e f o r ci n g f i e l d.T he w at er t em pe ra tu re o ve r t he s he lf i s a ls o s tr on gl yd et er mi ne d b y t he c ir cu la ti on t hr ou gh t he a dv ec ti on o f l ow - te mp e ra t ur e w a te r f r om t he L AT EX s he lf i n to t heTAV E s he l f. M o de l t e mp e ra t ur e s h av e a g oo d a gr e em e nt w i t h o b s e r va t i on s ( F i gu r e s 6 a n d 1 3 ) . F o r t h e L AT E X s h el f ,t h e a g re e m e nt f o r t h e N o v e m b er s e a s u r f a c e t e m p e r at u r e i sw i th i n o n e s t an d ar d d e vi a ti o n f r om t h e o b se r va t io n s

r e p or t e d b yNowlin et al. [ 19 9 8, F i g ur e 4 . 2 1 6 ] .[34] T he A CW SC a ls o c on tr ib ut es t o t he s ea so na l v ar i-

a b i li t y o f t h e v e r t i c al s t r uc t ur e o v er t h e TAV E s h el f . F r o mA p ri l t o A u gu st , t h e o f fs h or e s ur f ac e E km a n t r an s po rt s o f l o w- s al i ni t y w a te r, t h e u pw e ll i ng o f d ee p w a te r o nt o t hes h el f ( F ig ur e 7 ) , a n d t h e p os i ti v e h e a t f l ux i n to t h e o c e an[Zavala-Hidalgo et al., 2 0 02 ] , i n cr e as e t he s t ra t if i ca ti o no v er t h e s h el f , w hi c h h as b e en r e po rt e d p r ev i ou sl y [Sotoand Escobar, 1 99 5] a nd i s s up p or t ed b y t he r e la t iv e ly l o ws u bs ur f ac e t e mp e ra t ur e o bs e rv e d i n t h e h i st o ri c al h yd r o-g r a p h i c d a t a ( F i g u r e 5 ) . T h e o f f s h o r e E k m a n c o m p o n e n t c a n

be distinguished in the mean surface currents (Figure 3).F r om S e pt e mb e r t o M a rc h, t he d o wn c oa st A C WS C f a vo r s

t he p il in g o f l ow -d en si ty w at er t ow ar d t he c oa st a nd t hec o as t al c ur r en t i s c on f in e d t o t h e i n ne r a n d i n te r me di a te

Figure 12. M o nt h l y m e a n c h l or o p hy l l a c on t en t ( m g m3) e s ti m at e d f r om S e aW iF S d a ta . M on t hl ym ea ns f or J an ua ry A pr il f ro m 1 99 7 t o 2 00 1, M ay O ct ob er f ro m 1 99 8 t o 2 00 2, a nd N ov em be r

D e ce m be r f r om 1 9 98 t o 2 00 1.



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s he lf . T he i ns ho re E km an t r an sp or t t og et he r w it h t hen eg at iv e s ur fa ce h ea t f lu x a nd t he i nc re as e o f t ur bu le nt k in et ic e ne rg y i n t he u pp er o ce an b y t he i nc re as e o f t hew i nd s t re ss m a gn i tu de [Zavala-Hidalgo et al., 2 00 2] , m ixt h e w a te r c o lu m n a n d g e ne r at e a h o ri z on t al g r ad i en t w i th

l o w -d e n si t y w a t er n e a r t h e c o a st ( F i gu r e 7 ) .[35] T he s e as on a l v a r ia b il i ty o f t h e s ea s ur f ac e h e i gh t i sm ai nl y c on fi ne d t o t he s he lf , a s c an b e s ee n i n t he c ro ss -s h e lf g r a di e n ts o f S S H a l o n g t h e w e s te r n G o M ( F i gu r e 1 4 ) .I t s s ea s on al r a ng e, m ea s ur e d a s t h e m a xi m um m i nu s t hem in im um v al ue s, i s l ar ge r a t t he c oa st t ha n a t t he o ut er s he lf , t he s he lf b re ak , o r t he s lo pe . A t 2 6N, the seasonalr a n g e i s 2 2 c m a t t h e c o a s t a n d 1 0 c m a t t h e s h e l f b r e a k ;a t 2 4N the range is 2 4 c m a t t h e c o a s t a n d 1 2 c m 5 0 k mo f f t h e c o a s t , a n d a t 2 0N is 1 8 c m a t t h e c o a s t a n d 6 cm4 0 k m o f f t h e c o a s t ( F i g u r e 1 4 ) . T h e s e r e s u l t s s h o w t h a t t h ec o as t al c ur r en t s w i th in t he s h el f a n d t h e a t mo sp he r ic s eal e v el p r e ss u r e a r e t h e m a j o r c o nt r i bu t o rs t o t h e s e a s o n al s e al e ve l s i gn al r a th e r t ha n a w e st e rn b ou n da ry c ur r en t a s w a s

proposed by Sturges and Blaha [1976], Blaha and Sturges[ 1 98 1] , a n d Sturges [ 19 9 3 ]. H o w ev e r, a l t ho u g h w e a ke r ,t h e r e i s a s e a s o n a l s i g n a l a t t h e s l o p e t h a t m a y b e g e n e r a t e d

by a wester n boundar y current as Sturges and Blahas u g g e s t e d , b u t t h e c a u s e s o f t h e S S H v a r i a b i l i t y a t t h e s l o p ea r e n o t a na l yz e d h e r e. T he p e ak o f S SH i s a ss o ci a te d w i tht h e t r a ns i t io n b e t we e n u p co a s t t o d o w nc o a st c u r re n t a n d t h ed i s pl a c em e n t o f t h e r e g io n s o f s t r on g e r c o nv e r ge n c e. S i nc et he S SH i n G al ve st on l ea ds b y o ne m on th t he p ea k o n t hel o c a t i o n s d o w n c o a s t , i n m o d e l a n d t i d e g a u g e s , i t s e e m s t h a t t he t ra ns it io n i s n ot s im ul ta ne ou s b ut s in ce t he m od el i sf o rc e d w i th m o nt hl y w i nd s t h at c a nn ot b e d ef i ni t iv e lyconcluded. Nowlin et al. [ 1 9 9 8 ] r e p o r t e d t h a t t h e d o w n c o a s t

t o u p c oa s t t r a n s it i o n i n t h e L AT E X s h e l f o c c u r s i n a p e r io do f s e v e r a l w e e k s a n d n o t s i m u l t a n e o u s l y a l o n g t h e s h e l f b u t

t h e y f o u n d t h a t t h e u p c oa s t t o d o wn c o as t t r a ns i t io n o c cu r salmost simultaneously.

[36] T he S SH m ax im um i n S ep te mb er N ov em be r a ndt he m in im um i n J ul y h av e a c le ar s ig na l i n a ll s ec ti on s.T he c ro ss -s he lf g ra di en ts o f S SH a t 2 6N, 24N, and

20

N, on the TAVE shelf, reverse along the year, as thea lo ng s he lf c ur re nt s d o. I n t he se s ec ti on s t he g ra di en tspoint inshore from September to April and offshore fromA pr il t o J ul y. O ve r t he w es te rn C am pe ch e B an k t heg r ad i en t s a re a l wa y s o ff s ho r e, b e in g s t ro ng e r i n J u ly a ndw ea ke r i n O ct ob er N ov em be r. I n t he s ec ti on a t 9 5. 5W,o n t h e L AT EX s h el f , t h e r e i s a s e co n d m a x im u m i n M a y J un e, a s i s o bs er ve d i n t he G al ve st on t id e g au ge d at a( F i gu r e 1 4 ) .

5. Conclusions

[37] W it h r e g ar d t o i t s c i rc ul a ti on , t h e w e st e rn s h el f o f t h eG o M c a n b e d i vi de d i n t h re e r eg i on s : t h e TAV E s h el f , t he

L AT EX s he lf , a nd t he w es te rn C am pe ch e B an k. O ve r t h e TAV E s h e lf , b e t we e n 2 6N and 19N, the currents runs o ut h wa r d f r om S e pt e mb e r t o M a rc h a nd n or t hw a rd f r omM a y t o A u g u s t , r e a c h i n g m o n t h l y m e a n s p e e d o f 0 . 7 0 m s1;t h e t r a n s i t i o n p e r i o d s a r e f r o m l a t e M a r c h t o A p r i l a n d f r o ml a te A u gu s t t o S ep t em b er. D u ri ng t h e f a ll a nd w i nt e r, l o w-s a l i n i t y w a t e r f r o m t h e M i s s i s s i p p i a n d A t c h a f a l a y a r i v e r s i sa dv ec te d a cr os s t he L AT EX s he lf t o t he TAV E s he lf ,d e ve l op in g f r on t s a l on g t h e o ut e r s h el f o r t he s h el f b r ea k.D u ri ng t h e s u mm e r s e as o n, w a te r f r om t h e TAV E s h el f i sa d v ec t e d o n t o t h e L AT E X s h el f . O n t h e w e s t e r n C a m p e c heB a n k, t h e l o w -f r e qu e n cy c i r cu l a ti o n i s u p co a s t t h r ou g h ou t t he y ea r. O n t he L AT EX s he lf , m os t o f t he y ea r t he re i s a

c y cl o ni c c i rc u la t io n, w i th a s t ro ng c o as t al c u rr e nt a l on gt he i nn er s he lf a nd a w ea ke r br oa d c ur re nt o ve r t he

Figure 13. M od e l m on t hl y m e an s e a s u r fa c e s a li ni t y a n d t e m p er at u re f or J u ly a nd N o ve m be r o n t h eL ou i si a na - Te xa s s h el f . B a th y me t ry c on t ou r l i ne s o f 2 5, 5 0, 1 00 , a nd 2 00 m a r e s ho w n.



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Figure 14. M o de l m o n th l y m e a n s e a s u r f a c e h e i g ht ( m ) a t t h e s e c t i on s i n d ic a t ed w i t h c o r r e sp o n di n gl e tt e rs i n F i gu r e 1 . ( b ot t om ) T he b at h ym e tr y o f e a ch s e ct i on .



18/19

o u te r s he l f. T he r e su l ts f r om t he m o de l s i mu l at i on s s ho w as t r o n g e r o u t e r s h e l f c u r r e n t i n A p r i l a n d M a y w h e n t h e r e i s ac on ve rg en ce i n t he s ou th er n Te xa s s he lf t ha t f ee ds t hec u rr e nt . I n s um m er , t h e c ur r en ts o n t h e L AT EX s he l f a reeastward.

[38] A t t he s ea so na l f re qu en cy, t he m ai n f or ci ng o f t hec i rc ul a ti o n o n t h e w e s t er n s h el f o f t he g u lf i s t he A C WS C .

I t s p a t te r n i s s i m il a r t o t h e l o c a l o c ea n c i r cu l a ti o n t h r ou g h-o u t t he y ea r. O ve r t he TAV E s he l f, t h e A C WS C i s s ou t h-w a rd f r om S ep t em b er t o M ar c h a nd n or t hw a rd f r om A p ri lt o A u gu s t, a s i s t he a l on g s h or e c u rr e nt . O v er t he L AT EXs he lf t he A CW SC i s a lw ay s d ow nc oa st , e xc ep t f or t hes um me r m on th s w he n i t i s v er y w ea k a nd s om et im esr ev er se s, a nd o n t he w es te rn C am pe ch e B an k i s u pc oa st t h r ou g ho u t t h e y e a r. T h e c o r re l a ti o n b e t we e n t h e A C W SCa nd t he a lo ng s ho re s ur fa ce c ur re nt s i s h ig h o ve r m os t o f t h e s h el f , e x ce p t f o r t h e c o n v e r ge n ce z o ne s , s u p po r t in g t h eh yp ot he si s t ha t t he l oc al w in d s tr es s i s t he m ai n f or ci ngm ec ha ni sm o f t he w es te rn s he lf o f t he G oM . T he l ar ge r c o rr e la t io n c o ef f ic i en t s a r e o n t he TAV E s he l f, w h er e t he

A CW SC i s s tr on ge r a nd h as g re at er v ar ia bi li ty a t t heseasonal scale.

[39] T he re a re t wo m ai n c on fl ue nc e r eg io ns a lo ng t hew es te rn s he lf o f t he G oM : o ne i n t he n or th we st , a ro un d26.5N, mainly observed from April to August, and theo th er i n t he s ou th er nm os t p ar t o f t he B ay o f C am pe ch e,

between 93W a nd 9 5. 5W, o bs e rv e d f r om S ep t em b er t oM a rc h. T he c a us e o f t he c on f lu en ce o f t h e s h e l f c ur r en t s i st h e c o n v e rg e n ce o f t h e A C W SC , d u e t o t h e c o m b i na t i on o f t he c on ca ve s ha pe o f t he w es te rn g ul f a nd t he w in dd i re ct i on . T he m a in c on s eq ue n ce o f t h e c on f lu e nc e i s t heg en er at io n o f o ff sh or e c ur re nt s, w hi ch a re n ot or i ou s i ns a t el l i te i m a ge s b e c au s e t h e se c u r re n t s a r e o f t en a s s oc i a te dw i th w a te r s o f h i gh c h lo r op hy l l a c o n t en t a n d c o n tr a s ti n gt e mp er a tu re . A l th o ug h i t h as b ee n o bs er v ed t h at o ff s ho r ec u rr e nt s a re a s so ci a te d w i th e pi s od i c p r oc e ss e s, h e re i t i sf o u nd t h a t t h e s e e v en t s h a ve a s e a so n a l m o d u l at i o n.

[40] T he o ff sh or e E km an t ra ns po rt a ss oc ia te d w it h t heu p co as t A C WS C f av o rs u p we l li ng d ur i ng t he s p ri n g a n ds u mm e r o n t h e TAV E s he l f. D u ri ng t h e f a ll a n d w i nt e r t hei n s ho r e s u r fa c e E k m an t r a ns p o rt f a v or s t h e d o w nw e l li n g o nt h e i n n e r s h e l f a n d t h e c o n f i n e m e n t o f t h e l o w - s a l i n i t y w a t e r t o t he c o as t .

[ 41 ] Acknowledgments. T hi s p ro je ct w as f un de d b y t he O NR s p o n so r e d D i s t r i bu t e d M a r i ne E n v i ro n m en t F o r ec a s t S y s t em , t h e O f f i ceo f N av al R es ea rc h S ec re ta ry o f t he N av y g ra nt a wa rd ed t o J am es J .O B ri e n, a nd t he N AS A O ff i ce o f E ar t h S ci e nc e. S i mu la t io ns w er e

p erfo rmed o n th e IBM SPs at th e Flo rid a State Un iv ersity an d th e Nav alO c ea n og r ap hi c O f fi c e. C om p ut e r t i me w a s p r ov i de d b y t h e D o D H i ghP e r f or m a nc e C o m pu t i ng M o d er n i z at i o n O f f i c e , a n d b y t h e F S U S c h o o l o f C o mp u ta t io na l S c ie n ce a n d I n fo r ma t io n Te c hn o lo g y. We w o ul d l i ke t ot h a n k W i l t o n S t u rg e s a n d R o s ar i o R o m er o - Ce n t en o , w h o m a d e i m p o r t a nt c o mm e nt s t h at i m pr o ve d t h is s t ud y. T h e a u th or s w i sh t o t h an k r e vi e we r A l e x i s L u g o- F e rn an d e z a n d o n e a n o ny m o us r e v i ew e r f o r t h e i r v a l u ab l es u gg es t i on s . S p ec i al t h an k s t o P a ul M a rt i n f r om S t en ni s S p ac e C e nt e r

NRL fo r h elp in g with th e imp lem en tat io n o f th e Go M n u meri cals i m ul a t io n , t o L u is S o to f r om I C ML - UN A M w h o p r ov i de d t h e h y dr o -g r ap h ic d a ta f r om t h e P R OB M EX a n d O G M E X p r oj e ct s , a n d t o C a rl o sI l l e s ca s a n d R a n u l f o R o d ri g u e z w h o p r o c e s s ed t h e m f o r t h i s s t u d y. J u a n I .G o nz al e z f r om C I CE S E k i nd l y p r ov i de d t h e m o nt h ly m e an t i de g a ug ed a ta . We t h an k A l ex L e e f o r h i s c o n t ri b ut i on s t o t h e a n a l ys i s o f t h e s e al e ve l d at a . T h e a u th o rs w ou l d l i ke t o t h an k t h e S e aW iF S P r oj e ct ( C od e9 70 .2 ) a nd t he D is tr ib ut ed A ct i ve A rc hi ve C en te r ( Co de 9 02 ) a t t heG o dd a rd S p ac e F l i gh t C e nt e r, G r ee n be l t , M D 2 0 77 1 , f o r t h e p r od u ct i on

a n d d i st r i bu t io n o f t h e d a ta . T h es e a c ti v it i es a r e s p on s or e d b y N A SA sM i s s i o n t o P l a n et E a r t h P r o g r am .

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Go n zalez, J.

seasonal circulation on the western shelf of the gulf of mexico using a high-resolution numerical...

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